During period of data transferring process in a communicating system, errors of transmission data or failures of not receiving the transmission data usually happen from time to time, which will influence normal work of the whole system. In order to ensure normal system operation, some systems that support re-transferring function will send the same data continuously until receiving an accurate response indicating the data has been received from opposite side. In a communication system in which data are transferred through Quadrature Amplitude Modulation (QAM), the transferred data is determined by the symbols adopted in constellation figure, in other words, during the data transferring process, the data in X axis corresponding to a symbol in constellation figure is transmitted first, and then the data in Y axis corresponding to the symbol is transmitted. Nevertheless, when number of bits modulated to a symbol is greater than or equal to 4, transmission reliability of different bits in the symbol may be different from one another. Because of which, the reliability difference will enlarged after several re-transferring processes, this will further reduce reliability of the entire data, and influence normal performance of the system at the same time. Any attempts to keep reliability of the data bits unchanged after every transferring process will reduce performance of the decoder because of different bit reliability within a symbol.
In order to equipoise and improve reliability of each bit within a symbol during re-transferring process and raise decoding performance, an enhanced Hybrid Automatic Repeat reQuest (HARQ) method based on signal constellation figure arrangement has been provided. The method modifies reliability of different bits within each symbol through changing constellation figure of high order modulation during each re-transferring process, so that consistence of the reliability among different bits within each symbol during different re-transferring processes is kept, and performance of the decoder that is based on the re-transferring processes is improved. However, both receiver and sender must store all the necessary constellation figures that may be used in this method, which needs large storage capacity, especially in case of high order modulations, because the higher the modulation order is, the more transformation constellation figures are needed, and the larger storage capacity is needed.
The fundamental principle for realizing the above-mentioned HARQ method is adopting different Gray code figures in each data re-transferring process. Taking high order modulation on 16QAM data as an example, there can be several variations of constellation figures in high order QAM. But because each symbol in 16QAM is composed of four bits, four different and symmetric constellation figures are generally adopted as constellation transferring figures during re-transferring process to meet the requirements, in which the four constellation figures are employed periodically. FIG. 1 to FIG. 4 show four selected rectangular 16QAM constellation figures which may be used in the HARQ method, wherein, i1 represents the highest bit of the symbol, q1 represents the 2nd bit of the symbol, i2 is the 3rd bit and q2 is the lowest bit.
It can be seen from FIG. 1 that change probability of i1q1 is smaller than that of i2q2, which shows the reliability of i1q1 is higher than that of i2q2 when transferring data with the constellation figures. Similarly, FIG. 2 shows i2q2 has a greater change probability than i1q1, and i2q2 has higher reliability than i1q1 while transferring data according to the constellation figures. In this way, adopting the selected constellation figures in turn at each transferring process can ensure reliability of each bit in the data after several re-transferring processes keeping consistent essentially. Table 1 shows the numbers of constellation figures adopted in each transferring process in this method and corresponding performance analysis:
Number ofTimes ofconstellationtransferringfigurePerformance analysisThe 1stFIG. 1Reliability of i1q1 is greater than that oftransferringi2q2The 2ndFIG. 2After changing mapping relationshiptransferringbetween i1q1 and i2q2, reliability of i2q2 isgreater than that of i1q1The 3rdFIG. 3Reliability of i1q1 is greater than that oftransferringi2q2; this transferring is inverse mappingof the 1st transferring i2q24thFIG. 4Reliability of i2q2 is greater than that oftransferringi1q1; this transfer is inverse mapping ofthe 2nd transfer i1q1LaterFIG. 1~FIG. 4The four constellation figures from 1 to 4transferringdefined above are employed periodicallyTable 1 Constellation Figures Used in Data Transferring and its Performance Analysis
It can be seen from the above analysis all of the possible mapping relationships for the selected constellation figures that may be used should be stored at both sending side and receiving side in order to transfer and modulate data correctly, which will tremendously increase the storage amount. As for higher order modulations, more constellation figures may be used and therefore larger storage capacity will be required.